This invention relates to a micromachined capacitive electrical component in general. In particular the invention relates to a capacitive transducer such as a condenser microphone. Such micromachined components or systems are often referred to as Micro Electro-Mechanical Systems (MEMS).
A capacitive transducer such as a condenser microphone typically has a thin diaphragm that is arranged in close proximity to a back plate defining an air gap therebetween. The thin diaphragm is constrained at its edges, so that it is able to deflect when sound pressure is acting on it. Together the diaphragm and back plate form an electric capacitor, where the capacitance changes when sound pressure deflects the diaphragm. In use, the capacitor will be electrically charged using a DC bias voltage. When the capacitance of the microphone varies due to a varying sound pressure, an AC voltage proportional to the sound pressure will be superimposed on the DC voltage. The AC voltage is used as output signal of the microphone.
The sensitivity of the microphone, ie the ratio of the output AC voltage to the input sound pressure acting on the microphone, increases with the applied DC bias voltage. Consequently, in order to obtain a highly stable sensitivity without drift in time, the DC voltage across the air gap between the diaphragm and the back plate must be very stable. Note that a highly stable sensitivity is a requirement for any critical application of microphones, such as for example microphones for sound level measurement or other technical or scientific purpose.
The DC voltage is applied from an external voltage source via a bias resistor. The bias resistance must be so high that it ensures a virtually constant charge on the microphone, even when the capacitance changes due to sound pressure acting on the diaphragm. The value of this bias resistor is typically 1 to 10 Gxcexa9. When the leakage resistance of the microphone is infinitely high, the voltage across the microphone equals the applied DC voltage. If however, the leakage resistance of the microphone is not infinitely high, the applied DC voltage is divided between the bias resistor and the leakage resistance of the microphone, and consequently, the sensitivity of the microphone decreases. Therefore, a usual and practical requirement for a highly stable microphone is that the leakage resistance must be at least 1000 times higher than the resistance of the bias resistor, even under severe environmental conditions, as for instance in conditions of high humidity and high temperature.
Another cause of a change in the voltage across the air gap between the diaphragm and the back plate is the presence of additional charges in the air gap, ie charges not related to an applied polarization voltage. This behavior is well known, and utilized in electret microphones, where an electric charge is intentionally stored in an insulator layer in the air gap, so an electrical field is present in the air gap of the microphone without the need for an external voltage supply. However, in condenser microphones that are polarized by an external voltage source, charge storage is undesirable, since it changes the DC voltage across the air gap, thus causing changes in sensitivity. Storage of charge in the air gap of the microphone requires the presence of an insulating layer in the air gap. So in a highly stable condenser microphone the presence of insulating layers between the diaphragm electrode and the back plate electrode is undesirable.
Summarizing, the construction of a condenser microphone with a highly stable sensitivity over time, requires:
1. A leakage resistance that is at least 1000 times the bias resistor value, even under severe environmental conditions
2. No insulating layers in the air gap between the diaphragm electrode and the back plate electrode
From traditional measurement condenser microphones for industrial and scientific purposes it is known that the leakage resistance is determined by the leakage current across the surface of an insulator disc that separates the electrical contacts of the connector of the microphone. Likewise, in micromachined condenser microphones, the leakage resistance is determined by leakage current across the surface of the insulating material that separates the diaphragm electrode and the back plate electrode. The leakage resistance increases if the shortest distance that the leakage current has to travel across the insulator is increased. In traditional measurement condenser microphones, the shortest distance is of the order of millimeters. In some of the micromachined condenser microphones that are presented in literature, the shortest distance comes down to the thickness of an insulator layer that is of the order of 1 xcexcm! This is for example the case in designs, where both the back plate and the diaphragm are made of monocrystalline or polycrystalline silicon, where a silicon dioxide spacer layer with a thickness between 1 and 3 xcexcm has to provide the electrical insulation between diaphragm and back plate. Examples of such constructions are presented in the publication entitled xe2x80x9cA silicon condenser microphone using bond and etch-back technologyxe2x80x9d by J. Bergqvist and F. Rudolf in the journal Sensors and Actuator A, 45 (1994) 115-124, and xe2x80x9cCapacitive microphone with low-stress polysilicon membrane and high-stress polysilicon back platexe2x80x9d by A. Torkkeli et al. in the journal Sensors and Actuator, 85 (2000) 116-123 (corresponding to U.S. Pat. No. 6,178,249, Hietanen et al.), and in U.S. Pat. No. 5,452,268 xe2x80x9cAcoustic transducer with improved low frequency responsexe2x80x9d. That type of construction cannot be expected to have a leakage resistance that is at least 1000 times the bias resistor value, especially under conditions of high humidity and temperature. Consequently, that type of microphone would be suitable only for uncritical low-end applications, but is definitely not suited for any critical application that requires the sensitivity to be stable over time.
Another microphone construction that may ensure a high leakage resistance between the diaphragm electrode and the back plate electrode is presented in the publication xe2x80x9cA new condenser microphone with a p+ silicon membranexe2x80x9d, by T. Bourouina et al. in the journal Sensors and Actuators A, 31 (1992) 149-152. That microphone is made by bonding a silicon part, containing an etched diaphragm, onto a glass substrate, that contains the back plate electrode. The shortest distance between the diaphragm electrode and the back plate electrode is now considerably larger than the air gap thickness, so a higher leakage resistance can be expected. However, a disadvantage of using chips made of bonded silicon- and glass substrates is the thermal mismatch between the two materials. Although glass types exist (e.g. Pyrex 7740) that are developed with the purpose of providing properties matching those of silicon, they never exactly match the thermal expansion coefficient over the complete operating range of the transducer (typically xe2x88x9230xc2x0 C. to +150xc2x0 C.). The difference in thermal expansion coefficient causes a thermal stress in the diaphragm, which gives increased temperature sensitivity. Another effect is thermal bending of the silicon-glass sandwich (like a bimetal, since the silicon and glass have a comparable thickness) that gives a temperature-dependent change in air gap thickness that also gives a change in sensitivity. Therefore, microphone chips made by bonding silicon to glass suffer from temperature drift of the sensitivity, which is undesirable in critical applications of microphones.
Microphone chip designs based on an insulating diaphragm material are often to be preferred from a fabrication point-of-view. There are several electrically conducting diaphragm materials that can be made on silicon wafers. In the table below, a list of conducting diaphragm materials is shown, together with the disadvantages.
With most of the conductive diaphragm materials, the stress cannot be controlled, whereas this is an extremely important parameter to control, microphone parameters such as sensitivity and resonance frequency. The stress of polycrystalline silicon can be controlled with sufficient accuracy, but the fabrication of microphone diaphragms is complicated, since the thin diaphragms have to be protected during etching of the silicon wafer. A very attractive insulating diaphragm material is silicon nitride. The stress of the silicon nitride layers can be accurately controlled, and the fabrication of diaphragms is relatively easy, since silicon nitride is hardly attacked by the silicon etchant. Therefore, we consider silicon nitride to be a better diaphragm material than the available conducting materials. Other suitable diaphragm materials are silicon oxynitride, and multi-layer diaphragm comprising two or more layers of silicon dioxide, silicon oxynitride or silicon nitride, respectively.
To obtain favorable mechanical properties of the diaphragm, insulating materials are often used as diaphragm material in micromachined condenser microphones. As a consequence, the diaphragm has to be provided with an extra metal layer, preferably on its surface or possibly as an intermediate layer. For technological reasons, this metallization is often done as a final step in the fabrication process, causing the metal layer to be on the outside of the microphone, and the insulating diaphragm material to be located between the diaphragm electrode and the back plate electrode. Examples of such microphones are found in the publications xe2x80x9cMiniature condenser microphone with a thin silicon membrane fabricated on SIMOX substratexe2x80x9d by P. Horwath et al. (Proc. Transducers ""95, Stockholm, Sweden, Jun. 25-29, 1995, pp. 696-699), xe2x80x9cAn integrated silicon capacitive microphone with frequency-modulated digital outputxe2x80x9d by M. Pedersen et al. (Sensors and Actuators A, 69 (1998) 267-275), xe2x80x9cFabrication of silicon condenser microphones using single-wafer technologyxe2x80x9d by P. R. Scheeper et al. (IEEE Journal of MEMS, 1 (1992) 147-154). An advantage of having an insulating layer between the diaphragm electrode and the back plate electrode is a high leakage resistance between the electrodes, since a leakage current has to travel through the insulator layer. However, our own experience with micromachined microphones with an insulating layer between the diaphragm electrode and the back plate electrode is that these microphones show a serious drift of sensitivity in time, irreproducible sensitivity due to an uncontrolled amount of charge in the insulator arising during fabrication, assembly, and use, and hysteresis in sensitivity and capacitance when switching between zero DC bias and normal DC polarization voltage. Therefore, we believe that micromachined microphones with an insulating layer between the diaphragm electrode and the back plate electrode are not suitable for critical applications that require a stable sensitivity over time.
The article xe2x80x9cA subminiature condenser microphone with silicon nitride membrane and silicon back platexe2x80x9d by Hohm and Hess in 1989 (J. Acoust. Soc. Am., 85 (1989) 476-480) discloses a microphone chip with a silicon nitride diaphragm that is metallized with evaporated aluminium, and where the aluminium electrode is inside the air gap. The back plate chip consists of an oxidized silicon wafer provided with an aluminium electrode. A microphone is assembled by putting together a diaphragm chip and a back plate chip. Since both the diaphragm and back plate electrodes are placed inside the air gap, there are no insulators present between them. The publication also shows a photograph of the back plate wafer, showing that the minimum distance between the electrodes is about 100 xcexcm. The microphone design of Hohm and Hess fulfills two important requirements for making microphones with a highly stable sensitivity. However, a disadvantage of the design that is presented by the authors, is the high on-chip parasitic capacitance, causing the microphone signal to be divided by a factor of 4.3, corresponding to a loss of sensitivity of nearly 13 dB. This loss of sensitivity gives a decreased signal-to-noise ratio, and can therefore not be compensated by simply amplifying the microphone""s output signal. Another disadvantage of the large on-chip parasitic capacitance is that the harmonic distortion of a condenser microphone increases with the parasitic capacitance, as demonstrated in the publication xe2x80x9cReduction of non-linear distortion in condenser microphones by using negative load capacitancexe2x80x9d by E. Frederiksen in BandK Technical Review no. 1 (1996) 19-31. Therefore, a microphone design with a high on-chip parasitic capacitance will show a poor performance, which makes it useless for any demanding application. It is therefore desirable to have a low on-chip parasitic capacitance.
The relatively large contact area between the metallized diaphragm chip and the back plate chip causes the parasitic capacitance in the design of Hohm and Hess. Reducing the parasitic capacitance in that design is thus a matter of reducing the area of one of the adjacent chip surfaces. Reducing the area of the diaphragm chip implies that the silicon frame that surrounds the diaphragm is weakened considerably, which is undesirable. Besides, a photograph of the assembled transducer in the publication of Hohm and Hess shows that the silicon frame of the tested microphones can hardly be made smaller. Reducing the area of the back plate chip is only possible using a back plate layout that is totally different from the design of Hohm and Hess. The fabrication of the microphone of the invention is not possible with the fabrication process that is described by Hohm and Hess. Therefore, we propose a new design that results in microphones with a stable sensitivity, and that overcomes the problems with on-chip parasitic capacitance.
Another disadvantage of the Hohm and Hess microphone is that the aluminium electrodes tend to oxidize, and oxides are capable of retaining charges that add to the charges created by the polarization voltage, whereby the sensitivity changes proportionally. It is therefore desirable to avoid oxidizing materials between the diaphragm electrode and the back plate electrode.
Much of what is stated above in relation to condenser microphones also applies to capacitive electrical components in general and to MEMS components in particular.
The object of the invention is to provide a micromachined capacitive electrical component and in particular a condenser microphone, which meets at least one of the following requirements and preferably all three:
1. A very high leakage resistance between the diaphragm electrode and the back plate electrode,
2. Storage or accumulation of electrical charges in the air gap should be avoided, and
3. A low on-chip parasitic capacitance to avoid loss of sensitivity and to keep harmonic distortion low.